1. Field of the Invention
This invention relates to the field of display palettes that serve to map logical pixel values into physical appearance values.
2. Description of the Prior Art
It is known to provide electronic apparatus, such as computers, with a display palette system that serves to allow selection of a set of working colours (or other appearance parameters) for display from all possible displayable colours. More particularly, they serve to map a logical pixel value (for example generated by a software program) to a physical appearance value (such as a component RGB intensity value) that can be used to drive a display device. Such display palette systems can provide a great deal of flexibility in terms of the pixel appearance that can be made to appear on the display.
It is known that systems incorporating a display palette system may operate in a number of different modes with different mappings between logical pixel values and physical appearance values being used in the different modes. An example of this would be changing the number of bits of each logical pixel value to achieve a data size appropriate to the image being displayed. If fewer bits are used for each logical pixel value then a smaller number of different physical appearance values may be specified. In order to deal with this, the display palette circuit is reprogrammed for each different mode with a different set of pixel mapping data being stored within the display palette circuit in each mode.
Different modes such as the above are usually selected before any processing or display starts and then remain current throughout that processing or display. An additional degree of sophistication that may be desired is the ability to reprogram the display palette during on going image display without interrupting that image display. One way this has been done in the past is to reprogram the palette mapping data during the vertical flyback time of a raster display system such that the palette mapping data is only changed when it was not being used. A problem with this is that if changes in the palette mapping data overrun the flyback period and occur at the start of a frame of image display then display distortion at the start of the image can occur (this phenomenon can be seen as image "sparkle" at the top of an image). A further problem is that the software or other control that is being used to change the palette mapping data must be synchronised with the vertical synchronising signal of the display such that the times when the display palette is not being used may be properly identified. This need to synchronise to the vertical synchronisation signal places a constraint on the system design that is in many cases unwelcome, e.g. requiring particular interrupt service response with a fixed time.
It is an object of the invention to address the above mentioned problems.